1. FIELD OF THE INVENTION
The present invention relates to an apparatus for analysis which employs electrons and in which rotary vacuum pump evacuates gas or air from a specimen exchange chamber, and, more particularly, to an apparatus for analysis which employs electrons and in which the evacuated side of the apparatus is prevented from being contaminated by a vacuum pump oil.
2. DESCRIPTION OF THE RELATED ART
Means for preventing the interior of a vacuum chamber in a vacuum system from being contaminated by an oil backstreaming from a vacuum pump employing oil is described on pp. 299 to 307 of THE JOURNAL OF VACUUM SCIENCE AND TECHNOLOGY, VOL. 8, No. 1 (issued in 1971). In a method described in this publication, on the basis of the fact that oil contamination increases rapidly when the pressure at the inlet of the oil-sealed rotary vacuum pump decreases below 13 Pa (0.1 Torr), a dried nitrogen purging portion is arranged at the inlet of the oil-sealed rotary vacuum pump and prevents the pressure of the inlet from decreasing below a level from 13 to 40 Pa (0.1 to 0.3 Torr)
The increase in magnification of analysis systems which employ electrons such as electron microscopes has been advanced in recent years, but the further increase has been arrested by the contamination of microscope columns by an oil backstreaming from the oil-sealed rotary vacuum pumps used to evacuate air from specimen exchange chambers. Particularly, when an oil-sealed rotary vacuum pump operates at the almost ultimate pressure of the pump, this backstreaming of oil becomes a high degree. However, the above-described conventional purging method maintains the pressure of the inlet of the vacuum pump above 13 Pa (0.1 Torr) by purging in order to prevent the interior of the vacuum chamber from being contaminated by the oil backstreaming to the vacuum chamber from the oil-sealed rotary vacuum pump. So, when this conventional purging method is applied to the electron microscope, the pressure in the specimen chamber from which unnecessary gas has been evacuated is a high degree, so that the pressure in the specimen chamber is not be decreased below 13 Pa (0.1 Torr) in order to maintain the clean environment of the specimen chamber.